An electrical double layer capacitor is required to be high in withstand voltage. The withstand voltage of an electrical double layer capacitor depends upon electrolyte. Conventionally, in order to obtain higher withstand voltage, trials and errors have been repeated. For example, many supposed solutes and solvents are combined to synthesize electrolyte, and the withstand voltage of the synthesized electrolyte is measured. When the result of withstand voltage measurement satisfies the intended withstand voltage, it is used as electrolyte for electrical double layer capacitors. In this way, the withstand voltage of capacitors is improved.
Also, for effectively utilizing such withstand voltage improvement of electrolyte, the capacitance is adjusted with respect to positive electrode and negative electrode to manufacture electrical double layer capacitors.
Such a conventional electrical double layer capacitor is disclosed for example in Japanese Patent No. 3405040 and Japanese Unexamined Publication No. 2004-71814.
However, electrolyte has an inherent withstand voltage. And, an electrical double layer capacitor cannot be used with a voltage higher than the inherent withstand voltage of the electrolyte used for it. This is because the deterioration of electrical characteristics such as reduction of capacitance and increase of internal resistance are accelerated, and also, the case becomes easier to break down due to electrolyte reaction accompanied by increase in the amount of gas generated. Also, it is possible to make the most of inherent withstand voltage of the electrolyte by adjusting the electrode capacitance of positive electrode and negative electrode, but even in that case, the capacitor cannot be used with a voltage higher than the inherent voltage of the electrolyte.